Apparatus and method for digital beam-forming with low-resolution quantization

ABSTRACT

An antenna arrangement configured for digital beam-forming of a transmit signal comprising; a number N&gt;1 of digital to analog converters, DACs, each of the N DACs being arranged to receive one respective digital transmit signal component, and to convert and output an analog transmit signal component, each of the N DACs having a respective resolution below a resolution required to fulfill a regulatory radio requirement in an interchangeable antenna arrangement arranged for transmission by a single antenna element connected to a single DAC; and N antenna elements, each of the N antenna elements being configured to receive one respective analog transmit signal component and to transmit the analog transmit signal component as part of the digitally beam-formed transmit signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 35 U.S.C. §371 National Phase Entry Applicationfrom PCT/EP2014/068225, filed Aug. 28, 2014, and designating the UnitedStates, which claims priority to U.S. Provisional application No.62/010,710, filed Jun. 11, 2014, the disclosures of which areincorporated by reference herein.

TECHNICAL FIELD

Disclosed herein are, for example, an apparatus and method for digitalbeam-forming with low-resolution quantization.

BACKGROUND

Very large antenna array systems, such as multiple input multipleoutput, MIMO, systems, provide the opportunity for spatial divisionmultiple access, SDMA, in which each individual user may be served withthe same time-frequency resource as other users. This multiple accesstechnique requires accurate and coherent control of both amplitude andphase over the antenna array, which is an issue often solved by the useof high speed, high resolution digital to analog converters, DACs.

FIG. 2 shows an antenna arrangement 200 using a fully digitalbeam-forming scheme, which requires one high resolution DAC perantenna-branch, shown in FIG. 2 as “High Res DAC”. The respective outputsignals from the DACs are processed by mixers fed by a signal from anoscillator, OSC, and amplified by power amplifiers, PA, before beingtransmitted via antenna elements. FIG. 2 illustrates the case of a4-dimensional antenna array, i.e., an antenna array with four antennaelements, but the concept is readily extendable to an arbitrary numberN>1 of antenna elements.

High speed, high resolution DACs tend to consume a significant amount ofpower. For some example DAC circuits, every added bit, i.e., increase inDAC resolution, doubles the chip-area and the power consumption of theDAC. State-of-the-art high speed and high resolution DACs capable ofrunning at high bandwidths usually consume power in the range of 1-2Watts, which, when multiplied by the number of antenna elements in alarge antenna arrangement, can result in a power consumption on theorder of several hundreds of Watt's.

A power-conserving alternative to the fully digital beam-forming schemediscussed in connection to FIG. 2 is analogue beam-forming. FIG. 3 showsan antenna arrangement 300 with an analog beam-forming mechanism usingphase-shifters, F, which in turn are controlled by a pre-coder thatoutputs control voltages c1, c2, c3, c4.

Analog beam-formers, such as the one 300 illustrated in FIG. 3, oftencomprise either passive structures such as a Butler matrices whichconstrains the spatial duplexing into a number of fixed beams, oranalogue phase-shifters, which requires advanced calibration, sincethese usually comprise nonlinear analogue components. Further, analogphase-shifters do not provide means for control of the amplitude of therespective output signals. In order to provide beam-forming withamplitude control of the different antenna signals, either the PAs needto be able to provide fine-grained amplitude control, or an additionalset of amplifiers are needed in the design. Both of which options add tothe complexity of the antenna arrangement.

Consequently, some present antenna arrangements configured for digitalbeam-forming of a transmit signal either consume significant amounts ofpower, or are constrained in their spatial duplexing ability.

SUMMARY

An object of the present disclosure is to provide at least an antennaarrangement, a network node, and methods which seek to mitigate,alleviate, or eliminate one or more of the above-identified deficienciesin the art and disadvantages singly or in any combination.

This object is obtained by an antenna arrangement configured for digitalbeam-forming of a transmit signal. The antenna arrangement comprises anumber N>1 of digital to analog converters, DACs. Each of the N DACs isarranged to receive one respective digital transmit signal component,and to convert and output an analog transmit signal component. Each ofthe N DACs has a respective resolution below a resolution required tofulfill a regulatory radio requirement in an interchangeable antennaarrangement arranged for transmission by a single antenna elementconnected to a single DAC. The antenna arrangement also comprises Nantenna elements. Each of the N antenna elements is configured toreceive one respective analog transmit signal component and to transmitthe analog transmit signal component as part of the digitallybeam-formed transmit signal.

In this way, the overall power consumption of the antenna arrangement isreduced due to the use of DACs with reduced resolution. Thus, a morepower efficient antenna arrangement is provided.

Also, the present antenna arrangement provides for more flexibilitycompared to the use of analogue phase-shifters and/or Butler-matrices,leading to improved spatial resolution for beam-forming and/orbeam-tracking.

Furthermore, there is by the present technique a reduced need for alarge oversampling ratio, OSR, compared to using low resolution DAC'sand low-order MIMO.

According to an aspect, the antenna arrangement further comprises ade-correlator arranged to receive N correlated digital transmit signalcomponents, and to de-correlate quantization errors in the N correlateddigital transmit signal components, and to output N digital transmitsignal components to the respective N DACs.

The de-correlator reduces correlation between distortion components inthe transmitted analog transmit signal components. In this way signaldistortion from the different antenna branches averages out at areceiver of the transmitted signal and consequently improvedtransmission conditions are obtained.

According to a further aspect, the de-correlator comprises N ditheringunits. Each of the N dithering units is configured to receive arespective correlated digital transmit signal component, to add adithering sequence to the received correlated digital transmit signalcomponent, and to output a digital transmit signal component based onthe added dithering sequence and the received correlated digitaltransmit signal component to a respective DAC.

Thus, on average, the low resolution DAC's are causing less interferencedue to the dithering sequences, since the dithering sequences serve tode-correlate quantization errors in the N correlated digital transmitsignal components.

The object is also obtained by a method, performed in an antennaarrangement configured for digital beam-forming of a transmit signal.The method comprises converting, by a number N>1 of digital to analogconverters, DACs, N digital transmit signal components into N respectiveanalog transmit signal components. The converting is here performed at arespective resolution below a resolution required to fulfill aregulatory radio requirement in an interchangeable antenna arrangementarranged for transmission by a single antenna element connected to asingle DAC. The method also comprises transmitting the N analog transmitsignal components via N antenna elements as a digitally beam-formedtransmit signal.

There is further provided herein computer programs comprising computerprogram code which, when executed in an antenna arrangement, causes theantenna arrangement to execute a method according to aspects disclosedherein.

The computer programs and the methods display advantages correspondingto the advantages already described in relation to the antennaarrangement.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features, and advantages of the present disclosure willappear from the following detailed description, wherein some aspects ofthe disclosure will be described in more detail with reference to theaccompanying drawings, in which:

FIG. 1 illustrates an exemplary wireless communication system accordingto some aspects.

FIG. 2 illustrates an antenna arrangement with high-resolution DACsaccording to prior art.

FIG. 3 illustrates an antenna arrangement with analog phase-shiftersaccording to prior art.

FIGS. 4-8 illustrate antenna arrangements according to aspects of thepresent teaching.

FIG. 9 is a block diagram of a base station according to some aspects.

FIG. 10 is a chart illustrating an error vector magnitude, EVM, vs. DACresolution.

FIG. 11 is a power spectral density, PSD, graph.

FIG. 12 is a flowchart illustrating aspects of method steps performed inan antenna arrangement.

FIG. 13 is a flow chart illustrating a process according to some aspectsof the present teaching.

DETAILED DESCRIPTION

The present teaching makes use of the scaling of consumed power of a DACwhich occurs when reducing the resolution of the DAC, i.e., the numberof bits, along with the “massiveness” of advanced large antenna arraysin order to reduce the overall power consumption of the antennaarrangement and still deliver a high fidelity link with low error vectormagnitude, EVM, in receivers such as user equipment, UE, or otherwireless devices.

The decreased resolution in terms of DAC bits further has a positiveeffect on the possibilities of integration of the antenna array, sincethe physical DAC chip-size may be reduced down to a fraction of its highresolution counterpart.

The increased interference and unwanted signal emissions, such asout-of-band emission, caused by the increase in quantization noise maybe averaged not only over time and frequency, but also over space due tothe spatial selectivity of beam-forming antenna arrays. This averagingeffect is, according to aspects of the present teaching, furtherenhanced by the introduction of independent dithering sequences at eachDAC.

Aspects of the present disclosure will now be described more fully withreference to the accompanying drawings. The devices, computer programsand methods disclosed herein can, however, be realized in many differentforms and should not be construed as being limited to the aspects setforth herein. Like numbers in the drawings refer to like elementsthroughout.

FIG. 1 illustrates aspects of a wireless communication system 100, whichincludes user equipments, UEs, i.e., wireless devices 106, 108, and 110in communication with base station 102. Base station 102 providescoverage for cell 104. The base station 102 is in communication with acontrol node or network node 114 via a network 112. The network node 114may be any network node such as a Radio Network Controller, RNC, aMobility Management Entity, MME, a Mobile Switching Center, MSC, or BaseStation Subsystem, BSS. The base station 102, according to aspects,operates using space division multiple access, SDMA, where, if thedistance between the wireless devices 106, 108, and 110 is more than aminimum distance, the base station may reuse the same time-frequencyresource for more than one wireless device.

Occasionally, herein, the non-limiting term UE is used. The UE hereincan be any type of wireless device capable of communicating with anetwork node or another UE over radio signals. The UE may also be aradio communication device, target device, device to device, D2D UE,machine type UE or UE capable of machine to machine communication, M2M,a sensor equipped with UE, iPad, tablet, mobile terminal, smart phone,laptop embedded equipped, LEE, laptop mounted equipment, LME, USBdongles, Customer Premises Equipment, CPE etc.

Also, according to some aspects, generic terminology such as “radionetwork node” or simply “network node” is used. The network node can beany kind of network node which may comprise a base station, radio basestation, base transceiver station, base station controller, networkcontroller, evolved Node B, eNB, Node B, relay node, access point, radioaccess point, Remote Radio Unit, RRU, Remote Radio Head, RRH, etc.

The various aspects herein are described using LTE concepts. However,the aspects are applicable to any radio access technology, RAT, ormulti-RAT system, e.g., LTE FDD/TDD, WCDMA/HSPA, GSM/GERAN, Wi Fi,CDMA2000 etc.

FIG. 2 shows an antenna arrangement 200 using a fully digitalbeam-forming scheme according to prior art.

FIG. 3 shows an antenna arrangement 300 with an analog beam-formingmechanism according to prior art.

FIGS. 2 and 3 were discussed above in the background section and willnot be further described here.

FIG. 4 shows an antenna arrangement 400 configured for digitalbeam-forming of a transmit signal TX. The antenna arrangement 400comprises a number N>1 of digital to analog converters, DACs, 404 a-404d. Each of the N DACs is arranged to receive one respective digitaltransmit signal component TX_(i), i=1, 2, . . . , N, and to convert andoutput an analog transmit signal component TXa_(i), i=1, 2, . . . , N.

As opposed to, e.g., the antenna arrangement 200 shown in FIG. 2, eachof the N DACs here has a respective resolution below the resolutionrequired to fulfill a regulatory radio requirement in an interchangeableantenna arrangement arranged for transmission by a single antennaelement connected to a single DAC.

This means that, on their own, each of the DACs of the antennaarrangement 400 has too low resolution in order to generate a cleanenough signal for use in, e.g., a telecommunications system such as thesystem 100 illustrated in FIG. 1. However, because a number N>1 of lowresolution DACs are used in an antenna array, the aggregate transmitsignal TX will still fulfil radio requirements when received at a givenreceiver.

The antenna arrangement 400 shown in FIG. 4 further comprises N antennaelements 412 a-412 d. Each of the N antenna elements is configured toreceive one respective analog transmit signal component TXa_(i), i=1, 2,. . . , N, and to transmit the analog transmit signal component as partof the digitally beam-formed transmit signal TX.

The antenna arrangement 400 shown in FIG. 4, according to aspects,comprises a large number of antenna branches, i.e., DACs and antennaelements. Thus, the four antenna branches shown, e.g., in FIGS. 4-8herein is merely an example number of antenna branches, and the presentteaching is readily extendable to any number N>1 of antenna branches.

According to some aspects, the advantageous effects of the presentteaching, at least in terms of received signal quality and consumedpower, increases with the number of antenna elements N. Thus, theantenna arrangement 400 makes use of the beneficial scaling of theconsumed power of a DAC which occurs when reducing the resolution of theDAC, i.e., the number of bits, and the “massiveness” of large antennaarrays in order to reduce the overall power consumption of the antennaarrangement and still deliver a high fidelity link with low error vectormagnitude, EVM, in receivers such as user equipment, UE, or otherwireless devices.

Herein, what constitutes a “low resolution DAC” is to be interpreted ina broad and relative sense. That is, no specific resolution in terms ofnumber of bits need necessarily be used to define high and lowresolution. Rather, a high resolution DAC is here interpreted as being aDAC with specification such that the DAC, on its own, is capable ofgenerating a signal which fulfils some regulatory radio requirement,e.g., as defined in a 3GPP standard such as the specification documentTS 36.104 V12.3.0. A low resolution DAC, on the other hand, is a DACwith insufficient resolution to be used on its own.

The particular regulatory radio requirement referred to above variesaccording to aspects of the present teaching, from requirements on,e.g., error vector magnitude, EVM, at a receiver of the transmittedsignal TX to requirements on adjacent channel power ratio, ACPR, andoversampling ratios, OSR.

Thus, according to some aspects, the regulatory radio requirementcomprises one or more requirements on an adjacent channel power ratio,ACPR, of the antenna arrangement 101, 400.

According to some other aspects, the regulatory radio requirementcorresponds to one or more radio performance requirements defined bythird Generation Partnership Project, 3GPP, specification document TS36.104 V12.3.0.

The DACs 404 a-404 d shown in FIG. 4 need not necessarily be of the sameresolution. Hence, according to some aspects, one or more of the N DACs404 a-404 d has a resolution different from one or more other DACs 404a-404 d comprised in the antenna arrangement 400.

The antenna arrangement shown in FIG. 4 is, according to some aspects,comprised in a network node such as the base station 102 shown in FIG.1.

FIG. 5 shows an antenna arrangement 500 comprising a de-correlator 501arranged to receive N correlated digital transmit signal componentsTX′_(i), i=1, 2, . . . , N, to de-correlate quantization errors in the Ncorrelated digital transmit signal components, and to output N digitaltransmit signal components TX_(i), i=1, 2, . . . , N to the respective NDACs 404 a 404 d.

Thus, the de-correlator reduces correlation between distortioncomponents, such as quantization errors, in the transmitted analogtransmit signal components. In this way signal distortion from thedifferent antenna branches averages out at a receiver of the transmittedsignal and consequently improved transmission conditions are obtained.

Consequently, the de-correlator 501, according to aspects, increases theperformance of the antenna arrangement 500 compared to the antennaarrangement 400 in that an aggregate error vector at a receiver of thebeam-formed transmit signal TX now has uncorrelated distortioncomponents with zero mean value and without bias. That is, suppose thetransmitted signal is TX=s+e, where s is the payload signal vector and eis an error vector dominated by DAC artefacts due to the reduced DACresolution, wherein the number of elements in s and e correspond to thenumber of antenna elements. Suppose further that a transmission channelbetween the antenna arrangement 500 and a receiver of the beam-formedtransmit signal TX can be modelled by a channel matrix H. The receivedsignal is then essentially given by y=H(s+e)+w=Hs+He+w, where w isdistortion vector added at the receiver, such as receiver noise.

Now, for a large antenna array, the length of vector e is significantlylarger than the length of vector y. Thus, when matrix H multipliesvector e, a weighted summation of the elements in e occurs.Consequently, if the elements of e are uncorrelated and zero mean, thepower of the term He will be small, i.e., a beneficial effect isobtained from averaging over vector e as long as the elements in vectore are uncorrelated and have zero mean. The de-correlator 501 providesthis beneficial feature.

FIG. 6 shows an antenna arrangement 600 further comprising a pre-coder601 arranged to receive a common digital transmit signal vectorTX_(dig), to pre-code the common digital transmit signal into Ncorrelated digital transmit signal components TX′_(i), i=1, 2, . . . ,N, and to output N correlated digital transmit signal componentsTX′_(i), i=1, 2, . . . , N, to the de-correlator 501.

The pre-coder 601, according to some aspects, acts as an encoder. Thepre-coder 601 then generates a digital sequence for each antenna branch.The digital sequence may be a sequence of bits that are associated witha beam direction.

According to some aspects, the de-correlator 501 is comprised within thepre-coder 601. Thus, the pre-coder 601 and the de-correlator 501 neednot necessarily be separate modules as shown in FIG. 6.

According to some other aspects, the common digital transmit signalvector TX_(dig) is a multi-user signal, meaning that one or moreseparate user signals are comprised in the common digital transmitsignal vector TX_(dig). Each separate user signal is, according toaspects, beamformed using a respective set of beam-forming weights,i.e., antenna element amplitudes and phases.

FIG. 7 illustrates an antenna arrangement 700 wherein the de-correlator501 comprises N dithering units 702 a-702 d. Each of the N ditheringunits is configured to receive a respective correlated digital transmitsignal component TX′_(i), to add a dithering sequence d_(i) to thereceived correlated digital transmit signal component, and to output adigital transmit signal component TX_(i) based on the added ditheringsequence d_(i) and the received correlated digital transmit signalcomponent TX′_(i) to a respective DAC 404 a-404 d.

The dithering sequences can be generated in real time, or stored in amemory, MEM 713. Thus, according to some aspects, one or more of therespective dithering sequences d_(i)=1, 2, . . . , N comprises one outof a number of pre-determined periodic sequences stored in a memorydevice, MEM 713, of the de-correlator 501.

The actual dithering sequences vary according to different aspects ofthe present teaching.

Thus, according to aspects, one or more of the respective ditheringsequences d_(i), i=1, 2, . . . , N comprises a dithering sequenceindependent from one or more other dithering sequences of thede-correlator 501. In this way at least part of the dithering sequencesd_(i), i=1, 2, . . . , N are statistically independent of each other.

According to other aspects, one or more of the dithering sequencesd_(i), i=1, 2, . . . , N are uniformly distributed over an intervalcorresponding to −LSB/2 to LSB/2, where LSB denotes a signal amplitudecorresponding to the least significant bit of one or more DACs 404 a-404d comprised in the antenna arrangement 700.

According to further aspects, one or more of the dithering sequencesd_(i), i=1, 2, . . . , N are distributed according to a normaldistribution with zero mean and a pre-determined standard deviationsubstantially equaling LSB/2, where LSB denotes a signal amplitudecorresponding to the least significant bit of one or more DACs 404 a-404d comprised in the antenna arrangement 700.

According to some aspects, one or more of the respective ditheringsequences i=1, 2, . . . , N comprises a pseudo-noise, PN, sequence.

According to some further aspects, one or more of the respectivedithering sequences d_(i), i=1, 2, . . . , N comprises an additive whiteGaussian noise, AWGN, sequence.

The use of low resolution DAC's with, in some aspects, eitherstatistically independent or otherwise tailored, i.e., customized,dithering sequences provides advantages; The decreased DAC resolutionimproves the array power efficiency, while the dithering enablesadditional improvement of the received EVM and handling of interferencecaused by the increased quantization noise resulting from the decreasedDAC resolution.

FIG. 8 illustrates aspects of an antenna arrangement 800 included in thebase station 102. The antenna arrangement 800 may use a fully digitalbeam-forming scheme with a low resolution DAC 404 a-404 d at each branchwhich is preceded by independently generated dithering sequences d_(i),i=1, 2, . . . , N. The dithering sequences may be independent anduniformly distributed over the resolution of the DAC such as −LSB/2 to+LSB/2. The dithering sequences, which are statistically independentfrom branch to branch, may be generated using a standard random numbergenerator. While FIG. 8 illustrates a 4-dimensional array, the antennaarray is readily extendable to an arbitrary number N of antennas.

According to some aspects, the antenna arrangement includes a pre-coder601 that acts as an encoder. The pre-coder 601 may generate a digitalsequence for each antenna branch. The digital sequence may be a sequenceof bits that are associated with a beam direction. A respectivedithering sequence d_(i) may be applied 702 a-702 d to each digitalsequence for each antenna branch. Each antenna branch may include arespective low-resolution DAC 404 a-404 d that receives a correspondingdigital sequence and converts the digital sequence to an analog signal.Each antenna branch may further include modulators such as mixers 806a-806 d that modulate a corresponding analog signal to a predeterminedfrequency in accordance with a sinusoidal signal received from anoscillator 810. Each antenna branch may also include one or moreantennas 412 a-412 d for transmitting signals to one or more UEs.

Aspects combine the use of low resolution DAC's with eitherstatistically independent or otherwise tailored (i.e., customized)dithering sequences. The decreased DAC resolution improves the arraypower efficiency, while the dithering enables additional improvement ofthe received EVM and handling of interference caused by the increasedquantization noise resulting from the decreased DAC resolution.

FIGS. 5-8 show the de-correlator 502 or the dithering units 702 a-702 dupstream from the DACs 404 a-404 d, i.e., the transmitted signal isfirst de-correlated and then converted into analog domain. It is,however, appreciated that the opposite order of processing is alsopossible. Consequently, an antenna arrangement where a digital transmitsignal is first converted into analog transmit signal components andthen de-correlated in analog domain is, according to some aspects,disclosed herein.

FIG. 9 is a block diagram of an embodiment of base station 102. As shownin FIG. 9, base station 102 may include or consist of: a computersystem, CS, 902, which may include one or more processors 955, e.g., ageneral purpose microprocessor and/or one or more circuits, such as anapplication specific integrated circuit, ASIC, field-programmable gatearray, a logic circuit, and the like; a network interface 903 for use inconnecting base station 102 to a network; and a data storage system 906,which may include one or more non-volatile storage devices and/or one ormore volatile storage devices, e.g., random access memory, RAM. The CS902 may be configured to perform the functionality of the encoder thatgenerates a digital sequence. In aspects where base station 102 includesa processor 955, a computer program product, CPP, 933 may be provided.CPP 933 includes or is a computer readable medium, CRM, 942 storing acomputer program, CP, 943 comprising computer readable instructions,CRI, 944. CRM 942 is a non-transitory computer readable medium, such as,but not limited, to magnetic media (e.g., a hard disk), optical media(e.g., a DVD), solid state devices (e.g., random access memory, flashmemory), and the like. In some aspects, the CRI 944 of computer program943 is configured such that when executed by computer system 902, theCRI causes the base station 102 to perform steps described below, e.g.,steps described below with reference to the flow charts and messageflows shown in the drawings. In other aspects, base station 102 may beconfigured to perform steps described herein without the need for acomputer program. That is, for example, computer system 902 may consistmerely of one or more ASICs. Hence, the features of the aspectsdescribed herein may be implemented in hardware and/or software.

The aspects provide overall reduced power consumption compared to theuse of high resolution DAC's, resulting in a more efficient antennaarray. The aspects further provide more flexibility compared to the useof analogue phase-shifters/Butler-matrices, leading to improved spatialresolution for beam-forming/beam-tracking. The aspects further removethe need for high OSR compared to using low resolution DAC's andlow-order MIMO. Furthermore, the interference of the low resolution DACis reduced with the dithering sequences.

FIGS. 10 and 11 illustrate the results of simulations using the antennaarrays and antenna arrangements of the present teaching. FIG. 10 is achart that illustrates the trade-off between received EVM versus thenumber of DAC bits. As illustrated in FIG. 10, as the number of DAC bitsis increased, the reduction in the EVM rapidly reaches saturation, wherethe EVM is limited by other factors such as phase-noise, power amplifiernonlinearities, etc. For example, as illustrated in FIG. 10, thesaturated level of EVM is essentially reached using 5 bits for the DACresolution.

FIG. 11 is a power spectral density, PSD, graph that shows thetransmitted PSD versus frequency versus direction of arrival, DOA. Asillustrated in FIG. 11, the out of band/out of angle emissions isaveraged by the dithering, creating a reduced and uniform distributionover frequency and space.

FIG. 12 is a flowchart illustrating aspects of method steps performed inan antenna arrangement. In particular, there is illustrated a method,performed in an antenna arrangement 101, 400, 500, 600, 700, 800configured for digital beam-forming of a transmit signal TX. The methodcomprises converting S5, by a number N>1 of digital to analogconverters, DACs, 404 a-404 d, N digital transmit signal componentsTX_(i), i=1, 2, . . . , N, into N respective analog transmit signalcomponents TXa_(i), i=1, 2, . . . , N. The converting being performed ata respective resolution below a resolution required to fulfill aregulatory radio requirement in an interchangeable antenna arrangementarranged for transmission by a single antenna element connected to asingle DAC.

The method also comprises transmitting S7 the N analog transmit signalcomponents TXa_(i), i=1, 2, . . . , N, via N antenna elements 412 a-412d as a digitally beam-formed transmit signal TX.

According to some aspects, the method further comprises de-correlatingS3 quantization errors in a number N>1 of correlated digital transmitsignal components TX′_(i), i=1, 2, . . . , N, into N digital transmitsignal components TX_(i), i=1, 2, . . . , N.

According to aspects, the de-correlating S3 further comprises ditheringS31, by N dithering units 702 a-702 d, each of the N correlated digitaltransmit signal components TX′_(i), i=1, 2, . . . , N, by adding arespective dithering sequence d_(i) to each correlated digital transmitsignal component.

According to further aspects, the method also comprises pre-coding S1 acommon digital transmit signal vector TX_(dig) into a number N>1 ofcorrelated digital transmit signal components TX′_(i), i=1, 2, . . . ,N.

FIG. 13 illustrates aspects of a process performed in a base station.The process may generally start at step SX1 to generate, by an encoderat the base station, a digital sequence for each antenna branch includedin a plurality of antenna branches of the base station. The processproceeds to step SX3 to convert, using a low resolution DAC, acorresponding digital sequence received from the encoder to an analogsignal. Step SX3 may be performed for each antenna branch. Furthermore,a dither sequence may be applied to the digital sequence prior toconverting to the analog signal. The process proceeds to step SX5 totransmit, by at least one antenna, one or more beams in accordance witha direction associated with the corresponding digital sequence. Step SX5may be performed for each antenna branch.

Additionally, there is also disclosed herein;

A base station comprising: a plurality of antenna branches; an encoderthat generates a digital sequence for each antenna branch included inthe plurality of antenna branches; and wherein each antenna branchincludes: a low resolution digital to analog converter, DAC, 404 a-404d, that converts a corresponding digital sequence received from theencoder to an analog signal, and at least one antenna 412 a-412 d thattransmits one or more beams in accordance with a direction associatedwith the corresponding digital sequence.

According to aspects, a respective dithering sequence is applied to eachdigital sequence.

According to aspects, each respective dithering sequence isindependently generated and uniformly distributed over the resolution ofeach DAC 404 a-404 d.

According to aspects, each respective dithering sequence is customizedto the respective digital sequence.

According to aspects, each DAC 404 a-404 d has a resolution less than orequal to 8 bits.

According to aspects, each DAC 404 a-404 d has a resolution less than orequal to 5 bits.

According to aspects, each antenna branch further includes a signalmodulator that modulates the analog signal to a predetermined frequency.

According to aspects, the base station transmits the one or more beamsto one or more mobile stations in accordance with spatial divisionmultiple access, SDMA.

Furthermore, according to some example embodiments, a base stationincludes a plurality of antenna branches. The base station furtherincludes an encoder that generates a digital sequence for each antennabranch included in the plurality of antenna branches. Each antennabranch includes a low resolution DAC that converts a correspondingdigital sequence received from the encoder to an analog signal. Eachantenna branch further includes at least one antenna that transmits oneor more beams in accordance with a direction associated with thecorresponding digital sequence.

According to some other example embodiments, a method performed in abase station includes generating, by an encoder at the base station, adigital sequence for each antenna branch included in a plurality ofantenna branches of the base station. For each antenna branch, themethod includes converting, using a low resolution DAC at the basestation, a corresponding statistically independent digital sequencereceived from the encoder to an analog signal. For each antenna branch,the method further includes transmitting, by at least one antenna, oneor more beams in accordance with a direction associated with thecorresponding digital sequence.

The description of the example embodiments provided herein have beenpresented for purposes of illustration. The description is not intendedto be exhaustive or to limit example embodiments to the precise formdisclosed, and modifications and variations are possible in light of theabove teachings or may be acquired from practice of various alternativesto the provided embodiments. The examples discussed herein were chosenand described in order to explain the principles and the nature ofvarious example embodiments and its practical application to enable oneskilled in the art to utilize the example embodiments in various mannersand with various modifications as are suited to the particular usecontemplated. The features of the embodiments described herein may becombined in all possible combinations of methods, apparatus, modules,systems, and computer program products. It should be appreciated thatthe example embodiments presented herein may be practiced in anycombination with each other.

It should be noted that the word “comprising” does not necessarilyexclude the presence of other elements or steps than those listed andthe words “a” or “an” preceding an element do not exclude the presenceof a plurality of such elements. It should further be noted that anyreference signs do not limit the scope of the claims, that the exampleembodiments may be implemented at least in part by means of bothhardware and software, and that several “means”, “units” or “devices”may be represented by the same item of hardware.

The various example embodiments described herein are described in thegeneral context of method steps or processes, which may be implementedin one aspect by a computer program product, embodied in acomputer-readable medium, including computer-executable instructions,such as program code, executed by computers in networked environments. Acomputer-readable medium may include removable and non-removable storagedevices including, but not limited to, Read Only Memory, ROM, RandomAccess Memory, RAM, compact discs, CDs, digital versatile discs, DVD,etc. Generally, program modules may include routines, programs, objects,components, data structures, etc., that perform particular tasks orimplement particular abstract data types. Computer-executableinstructions, associated data structures, and program modules representexamples of program code for executing steps of the methods disclosedherein. The particular sequence of such executable instructions orassociated data structures represents examples of corresponding acts forimplementing the functions described in such steps or processes.

In the drawings and specification, there have been disclosed exemplaryembodiments. However, many variations and modifications can be made tothese embodiments. Accordingly, although specific terms are employed,they are used in a generic and descriptive sense only and not forpurposes of limitation, the scope of the embodiments being defined bythe following claims.

The invention claimed is:
 1. A power efficient first antenna arrangementconfigured for digital beam-forming of a transmit signal, the firstantenna arrangement comprising; a number N>1 of digital to analogconverters (DACs) including a first DAC and a second DAC, each of the NDACs being arranged to receive one respective digital transmit signalcomponent, and to convert and output an analog transmit signalcomponent; and N antenna elements, each of the N antenna elements beingcoupled to just one of the N DACs such that each of the N antennaelements is configured to receive the analog transmit signal componentoutputted by the DAC to which the antenna element is coupled, whereinthe N antenna elements are further configured to generate a digitallybeam-formed transmit signal by having each of the N antenna elementstransmit the received analog transmit signal component as part of thedigitally beam-formed transmit signal, wherein the first DAC isconfigured to have a first low resolution such that if the first DACwere used in a second antenna arrangement that utilized only one antennaelement to transmit a first analog signal produced by the first DAC,then the first analog signal transmitted by the second antennaarrangement would not fulfill a regulatory radio requirement, the secondDAC is configured to have a second low resolution such that if thesecond DAC were used in a third antenna arrangement that utilized onlyone antenna element to transmit a second analog signal produced by thesecond DAC, then the second analog signal transmitted by the thirdantenna arrangement would not fulfill said regulatory radio requirement,and said digitally beam-formed transmit signal fulfills said regulatoryradio requirement.
 2. The antenna arrangement according to claim 1,further comprising a de-correlator arranged to receive N correlateddigital transmit signal components, to de-correlate quantization errorsin the N correlated digital transmit signal components, and to outputthe N digital transmit signal components to the respective N DACs. 3.The antenna arrangement according to claim 2, further comprising apre-coder arranged to receive a common digital transmit signal vector(TXdig), to pre-code the common digital transmit signal into Ncorrelated digital transmit signal components, and to output Ncorrelated digital transmit signal components to the de-correlator. 4.The antenna arrangement according to claim 2, wherein the de-correlatorcomprises N dithering units, each of the N dithering units beingconfigured to receive a respective correlated digital transmit signalcomponent (TX'i), to add a dithering sequence (di) to the receivedcorrelated digital transmit signal component, and to output a digitaltransmit signal component (TXi) based on the added dithering sequence(di) and the received correlated digital transmit signal component(TX'i) to a respective DAC.
 5. The antenna arrangement according toclaim 4, wherein one or more of the respective dithering sequencescomprises a dithering sequence independent from one or more otherdithering sequences of the de-correlator, at least part of the ditheringsequences thus being statistically independent of each other.
 6. Theantenna arrangement according to claim 4, wherein one or more of thedithering sequences is uniformly distributed over an intervalcorresponding to −LSB/2 to LSB/2, where LSB denotes a signal amplitudecorresponding to the least significant bit of one or more of the N DACscomprised in the antenna arrangement.
 7. The antenna arrangementaccording to claim 4, wherein one or more of the dithering sequences isdistributed according to a normal distribution with zero mean and apre-determined standard deviation substantially equaling LSB/2, whereLSB denotes a signal amplitude corresponding to the least significantbit of one or more of the N DACs comprised in the antenna arrangement.8. The antenna arrangement according to claim 4, wherein one or more ofthe dithering sequences comprises a pseudo-noise, PN, sequence.
 9. Theantenna arrangement according to claim 4, wherein one or more of thedithering sequences comprises an additive white Gaussian noise, AWGN,sequence.
 10. The antenna arrangement according to claim 4, wherein oneor more of the dithering sequences comprises one out of a number ofpre-determined periodic sequences stored in a memory device of thede-correlator.
 11. The antenna arrangement according to claim 1, whereinthe regulatory radio requirement comprises one or more requirements onan adjacent channel power ratio (ACPR) of the antenna arrangement. 12.The antenna arrangement according to claim 1, wherein the regulatoryradio requirement corresponds to one or more radio performancerequirements defined by third Generation Partnership Project, 3GPP,specification document TS 36.104 V12.3.0.
 13. The antenna arrangementaccording to claim 1, wherein one or more of the N DACs has a resolutiondifferent from one or more other DACs comprised in the antennaarrangement.
 14. A network node comprising the antenna arrangementaccording to claim
 1. 15. A method, performed in a first antennaarrangement, the method comprising; employing N number of digital toanalog converters (DACs), wherein N >1, such that each of the N DACsconverts a respective digital transmit signal component to produce arespective analog signal component and outputs the respective analogtransmit signal component; and using N antenna elements, N>1, togenerate a digitally beam-formed transmit signal by having each one ofthe N antenna elements transmit one of the respective analog transmitsignal components as part of the digitally beam-formed transmit signal,wherein the N number of DACs includes a first DAC and a second DAC, thefirst DAC is configured to have a first low resolution such that if thefirst DAC were used in a second antenna arrangement that utilized onlyone antenna element to transmit a first analog signal produced by thefirst DAC, then the first analog signal transmitted by the secondantenna arrangement would not fulfill a regulatory radio requirement,the second DAC is configured to have a second low resolution such thatif the second DAC were used in a third antenna arrangement that utilizedonly one antenna element to transmit a second analog signal produced bythe second DAC, then the second analog signal transmitted by the thirdantenna arrangement would not fulfill said regulatory radio requirement,and said digitally beam-formed transmit signal fulfills said regulatorradio requirement.
 16. The method according to claim 15, furthercomprising; de-correlating quantization errors in N correlated digitaltransmit signal components to generate the N digital transmit signalcomponents.
 17. The method according to claim 16, the de-correlatingfurther comprising; dithering, by N dithering units, each of the Ncorrelated digital transmit signal components by adding a respectivedithering sequence (di) to each correlated digital transmit signalcomponent.
 18. The method according to claim 16, further comprising;pre-coding a common digital transmit signal vector into a number N>1 ofcorrelated digital transmit signal components.
 19. A computer programproduct comprising a non-transitory computer readable medium storingcomputer program code for performing the method of claim
 15. 20. Thefirst antenna arrangement of claim 1, wherein the N antenna elementsinclude a first antenna element and a second antenna element, the firstantenna arrangement further comprises N mixers including a first mixerand a second mixer, the first antenna element is coupled to the firstDAC via the first mixer, and the second antenna element is coupled tothe second DAC via the second mixer.
 21. The first antenna arrangementof claim 1, wherein the regulatory requirement is an error vectormagnitude (EVM) requirement.
 22. The first antenna arrangement of claim1, wherein the regulatory requirement is an adjacent channel powerration (ACPR) requirement.
 23. The first antenna arrangement of claim 1,wherein the regulatory requirement comprises one or more radioperformance requirements defined in the third Generation PartnershipProject (3GPP) specification TS 36.104 v12.3.0.